1. Field of the Invention
The present invention relates to the configuration of an ATM multiplexing/demultiplexing apparatus for converting time division multiplexed signals to ATM cells or converting ATM cells to time division multiplexed signals, and relates to the configuration of an ATM communication network using those apparatuses.
2. Description of the Related Art
Introduction of communication apparatuses and communication networks using the asynchronous transfer mode (ATM) is under way in the form of interconnection between existing communication networks and replacement of communication networks. In such an introduction form, input/output signals of the ATM communication networks and communication apparatuses are existing signals such as DS1 signals of 1.544 MHz. In the ATM communication networks, the DS1 signals are converted to ATM cells to undergo processing such as transmission and exchange.
Even if signals at a source node such as a subscriber terminal are already ATM signals, signals inputted to an ATM communication network via an existing DS1 transmission line and a communication network arrive at the communication network in such a state that an ATM cell (53 bytes) of the source node is divided and inserted into payloads of a DS1 signal. In order to restore the ATM cell for processing, the ATM communication network includes a cell multiplexing apparatus as described in "Study on ATM cell multiplexing apparatus--development of ATM cell processor," Exchange System SSE-94, Technical Report, The Institute of Electronics, Information and Communication Engineers, and JP-A-7-321819. And cell synchronizing processing is conducted. In other words, a predetermined field (such as a header error control (HEC) field) contained in the header portion of an ATM signal taken out of a DS1 signal is detected and an ATM cell is regenerated. Furthermore, according to the configuration of the ATM communication network, ATM cells are multiplexed as occasion demands and are transmitted to the communication network.
On the other hand, in the case where conventional non-ATM signals such as audio signals supplied from a telephone terminal arrive at an ATM communication network, the non-ATM signals are converted there to ATM cells according to the classification of the signals as described in the aforementioned papers. Specifically, DS1 signals such as audio signals transferred and processed at a fixed bit rate are put into payloads of ATM cells according to the arrangement of AAL Type 1 (ATM Adaptation Layer Type 1) determined by the recommendation I. 363 of the International Telecommunication Union (ITU-T). In addition, predetermined headers are added. Thus the DS1 signals are converted to ATM cells. As occasion demands, ATM cells are multiplexed and transmitted to the communication network. By the way, in transmitting and receiving audio signals, synchronization of the signal rate at the source node and the destination node is important. In order to assure synchronization even if an asynchronous ATM communication network lies on the way, a concept called synchronous residual time stamp has been introduced in the above described recommendation. According to this concept, information relating to a frequency difference between the source node and the communication network (residual time stamp RTS) is inserted into ATM cells transferred in an ATM comunication network between the source node and the destination node. On the basis of this infomation, a clock synchronized to that of the source node is regenerated on the receiving side to assure synchronization. To be concrete, a P-bit counter having four bits is activated in the source node with an X-divided clock fox (2.43 MHz) obtained by applying frequency division to a clock fo (155.52 MHz) of the ATM network as described in U.S. Pat. No. 5,260,978. In addition, an N counter for conducting 3008 counts is stepped by using a clock fs (1.544 MHz) of a transmitted signal (DS1). Whenever, the N counter has counted the fs clock 3008 times, the output of the P-bit counter is outputted to the destination node as the RTS at a period equivalent to once every eight ATM cells. On the receiving desctination, an oscillator such as a voltage-controlled quartz-crystal oscillator is controlled on the basis of the RTS to regenerate a clock fr synchronized to the clock of the transmitting side.
If the scale of a network becomes large due to an increase of subscribers or the like in an existing communication network, signals transferred in the network are typically multiplexed and transmitted at a higher rate in order to reduce the cost of the network configuration. For example, a configuration as desribed in recommendation T1. 107 of the American National Standards Institute (ANSI) is known. In this configuration, four DS1 signals are stuff-multiplexed into one DS2 signal. Furthermore, seven DS2 signals are stuff-multiplexed into one DS3 signal. Twenty eight DS1 signals are thus multiplexed into one DS3 signal for transmission. In other words, in such a configuration that ATM communication networks are put in existing communication networks, input/output lines of the ATM communication networks carry in many cases DS3 signals instead of DS1 signals described above.
The above described convertion in the ATM communication network between ATM cells and signals is conducted on the assumption that signals are DS1 signals. In the case where multiplexed signals such as DS3 signals are inputted to/outputted from the ATM communication network, therefore, it becomes necessary to demultiplex a DS3 signal to DS1 signals on the input side temporarily and stuff-multiplex DS1 signals into a DS3 signal again on the output side. If a stuff multiplexing/demultiplexing apparatus such as a conventional D31 multiplexing/demultiplexing apparatus is simply introduced in a stage preceding or succeeding the ATM communication network, however, the hardware scale becomes large, resulting in impaired economical efficiency. In addition, a synchronizing method using the above described SRTS is adopted in the ATM communication network depending upon the property of the signal to be handled (such as the audio signal described before). Therefore, information relating to the stuff possessed by the DS3 signal inputted to the ATM communication network is terminated on the source side of the ATM communication network. In other words, this stuff information cannot be used when conducting multiplexing into the DS3 signal again on the distination side of the ATM communication network. As described above, therefore, the clock is regenerated so as to correspond to the DS1 signal on the basis of the RTS. By using this clock, new stuff synchronous multiplexing is conducted. Due to such a configuration, hardware such as an oscillator for regenerating the clock becomes necessary for each DS1, resulting in further impaired economical efficiency.